E4332: VLSI Design Laboratorynagendra/E4332/2005/handouts/amradio-trf.pdf · E4332: VLSI Design Laboratory Nagendra Krishnapura ... Full wave rectifier with differential inputs

E4332: VLSI Design Laboratory

Nagendra KrishnapuraColumbia University

Spring 2005: [email protected]

1Nagendra Krishnapura: VLSI Design Laboratory, Spring 2005

AM radio receiver AM radio signals: Audio signals on a carrier Intercept the signal Amplify the signal Demodulate the signal-recover the audio Amplify the audio to drive a speaker


AM signal basics: time domain

Envelope(peak) of the carrier is the message


Sidebands around the carrier


AM signal basics: frequency domain

AM signals

Sidebands around the carrier


Broadcast AM signals

Broadcast AM channels 10kHz from each other


Receiver bandwidth

Receiver bandwidth must be constant


Receiver bandwidth

fc/f

bw=53 at the lowest end

fc/f

bw=161 at the highest end

High Q (~ quality factor) Maintain constant bandwidth


Receiver sensitivity and selectivity

Sensitivity: ability to detect small signals– AM radio sensitivity: ~50uV signals with 30%

modulation

Selectivity: ability to reject adjacent signals

– Dictated by the choice of architecture in our case


Tuned Radio Frequency(TRF) receiver

Input tuned circuit is the only filter providing selectivity

Coil on a ferrite rod


TRF receiver: input tuning


Resonant frequency(radians per second) ωo

= 1/sqrt(LCtune

)

3dB bandwidth ωb

Quality factor Q = ω0/ω

b

Series loss: Qs = 1/R

ssqrt(L/C)

– Bandwidth = Rs/L

Parallel loss: Qp = R

psqrt(C/L)

– Bandwidth = 1/CRp


2nd order filter basics

Resonant frequency(1/sqrt(LCtune

)) varies

from 530 to 1610kHz, approx 3x Fixed L, ⇒ C

tune varies by 9x

Series loss(Rs) only

– Bandwidth = Rs/L

– No change with Ctune

Parallel loss(Rp) only

– Bandwidth = 1/Ctune

Rs

– varies by 9x with change in Ctune



Some bandwidth variation with tuning– Bandwidth < 10kHz at low end– Bandwidth > 10kHz at high end

2nd order filter. Limited out of band attenuation

⇒ Poor selectivity in a TRF receiver Suggestion: Use a very large on chip R

p to

maintain as high a Q as possible



High impedance input necessary– Source follower buffer– Differential amplifier


TRF receiver: Input amplifier

Use large resistors for input biasing






TRF receiver: Detector

No diodes in CMOS process Input amplitude > diode drop Use of an amplifier in feedback to improve

sensitivity


AM radio: specifications Signal levels: Input from 50µV to 5mV RF amplifier with AGC Output of RF amplifier with AGC from 50mV

to 200mV– Max. gain = 50mV/50µV = 1000 (60dB)– Min. gain = 200mV/5mV = 40 (32dB)– Total gain variability = 1:25 (28dB)

Detector must work with 50mV-200mV inputs

Audio output max. ~ 1Vpk

into 8Ω speaker


AM radio: specifications Misc.: Supply voltage: 4.2-4.5V

– Operation with 3x 1.5V batteries– Try to design for 4.2V


AM radio: input signal generation

Use A from 50µV to 5mV Parameterized subcircuit(using pPar(“m”),

pPar(“A”) etc.) to make an AM source in Cadence


Amplifier basics


Gain = gm(R

L+1/g

ds) ~ g

mR

L

Gm = sqrt(µCox

/2*W/L*I0) = I

0/V

GS-V

T

Gain = gmR

L = I

0R

L/V

GS-V

T

– To change gain, I0R

L (the dc voltage drop across

RL) or V

GS-V

T (related to transistor current

density) has to be changed Linearity improves with increasing V

GS-V

T

– Amplifier: larger VGS

-VT

– Switch: smaller VGS

-VT

Amplifier basics


RF amplifier I


AC coupled to remove offsets Single ended input/output-simple Gain = g

mR

L/2(Analyze this!)

Ac coupling resistors: pMOS transistors Ac coupling corner frequency: ~ 1dB

attenuation at lower end of AM band Capacitor values: 5pF or less

– Linear capacitor density ~ 0.9fF/µm2

Resistor values: upto 10kΩ– Resistivity ~ 800Ω/sq.

RF amplifier I


RF amplifier II


AC coupled to remove offsets Single ended input/output-simple Gain = g

mR

L(Analyze this!)





RF amplifier II


RF amplifier III


AC coupled to remove offsets Differential stages

– 2x ac coupling capacitors Gain = g

mR

L(Analyze this!)





RF amplifier III


Detector I


Implement vi(t)*sgn(v

i(t)) and filter the result

– Full wave rectification and filtering Filtering capacitor C

– retain audio, remove RF– External, if too large

Upper pair should act as a switch: sgn(vi(t))

Lower pair should act as a linear amplifier(over the entire range of input signals)

Detector I


Detector II


Full wave rectifier with differential inputs Half wave rectifier with single ended inputs Followed by amplifier and filter Filtering capacitor C

– retain audio, remove RF– External, if too large

Detector II


Peak detector

T audio , max ≫RC ≫T RF , min


Detector III


Detector III

Single stage “op amp”


Peak detector Discharge time constant slower than

charging time constant– Negligible discharge between RF cycles– Full discharge between audio cycles

Detector III


Audio amplifier

Feedback for linearity Output current ~ 1V/8Ω = 125mA


Class A opamp

Need to bias with I0 = 125mA!


Class AB opamp

Vb1

and Vb2

adjusted so that output branch

current is I0


Class AB opamp

Output pMOS gate pulled down to drive out a large current ( > bias)


Class AB opamp

Output nMOS gate pulled up to pull in a large current (> bias)


Class AB opamp: bias generation


Class AB opamp: full schematic


Bias current generation

• One external resistor to fix bias currents

– Bias line coupling can lead to problems in high gain multistage circuits



Separate mirroring branch for each stage

+ Reduced interstage coupling

– Increased current in bias branches



RC filter to each biasing MOS transistor

+ Reduced interference and noise

+ No increase in bias currents


Passive components


Passive components: Resistors

Passive resistor– Min. width=; few kOhms

MOS resistors– Need bias– Greater range of values– Voltage tunability


Passive components: capacitors

Passive (poly1-poly2)– Few pF– Bottom plate to ground parasitic

MOS capacitors– Higher capacitor density– Need to be biased in strong inversion

Documents

E4332: VLSI Design Laboratorynagendra/E4332/2005/handouts/amradio-trf.pdf · E4332: VLSI Design Laboratory Nagendra Krishnapura ... Full wave rectifier with differential inputs